I fatten mine on a mixture of potatoes and whole barley, and at this time of year they get the old peas (cooked) that got missed when picking, plus bread crusts when we get to the end of a loaf and of course whatever they find when free ranging.
Ours are dispatched from 16 weeks to about 20 weeks, after that they tend not to put on much more weight for the food they eat and also tend to get a bit too tough.

We feed them as many boiled seed patatoes, bread and "broken grains" as the will eat. Never leaving there feeding bowls empty.

The bread i buy from the local bakery, its about two days old before we get it. But then it only costs 1 euro for a 50 liter bag full of breads and "bakery scraps" i think bread is an excellent fattener.

I read once that the famous (Around here that is) Poule de Bresse chickens are fattened using milk powder and wheat flower. I intend to try this way of fattening coming spring.

All the above,any maize adds colour.I like the boiled mash pots as it's easy to incorporate other stuff into it.If your'e raising the white hybrids (which I know you personally aren't)you must kill them before 12 weeks cos they put on weight so quick their heart can give out.